Doc:(issue_1593) Add flag category topic in docs (v2-maint)

docs/v2/examples/flags.md

@@ -230,6 +230,57 @@ That flag can then be set with `--lang spanish` or `-l spanish`. Note that
 giving two different forms of the same flag in the same command invocation is an
 error.
 
+#### Grouping
+
+You can associate a category for each flag to group them together in the help output, e.g:
+
+```go
+package main
+
+import (
+	"log"
+	"os"
+
+	"github.com/urfave/cli/v2"
+)
+
+func main() {
+	app = &cli.App{
+		Flags: []cli.Flag{
+			&cli.BoolFlag{
+				Name:     "silent",
+				Aliases:  []string{"s"},
+				Usage:    "no messages",
+				Category: "Miscellaneous:",
+			},
+			&cli.BoolFlag{
+				Name:     "perl-regexp",
+				Aliases:  []string{"P"},
+				Usage:    "PATTERNS are Perl regular expressions",
+				Category: "Pattern selection:",
+			},
+		},
+	}
+
+	if err := app.Run(os.Args); err != nil {
+		log.Fatal(err)
+	}
+}
+```
+
+Will result in help output like:
+
+```
+GLOBAL OPTIONS:
+   Miscellaneous:
+
+   --silent, -s  no messages (default: false)
+
+   Pattern selection:
+
+   --perl-regexp, -P  PATTERNS are Perl regular expressions (default: false)
+```
+
 #### Ordering
 
 Flags for the application and commands are shown in the order they are defined.